Lowering river nitrogen load requires major action
Assessing trends in nitrogen (N) loss is much trickier than exploring how erosion and sediment transport in Iowa have been affected by changing precipitation regimes and extreme events. This article looks at studies of the Raccoon River Watershed in Iowa.
Although a lot of N data is available for the Raccoon River, the data is spotty until 1974, quite a while after large changes in river N loads began (load is the total amount of a substance transported by the river over a defined time period). And because nitrogen cycles into and out of the environment and is consumed by plants and microorganisms, linking river N levels to specific actions on the landscape is very difficult.
We know that production in the corn-soybean system is maximized when the landscape is essentially saturated with nitrogen. Because the most common form of N in the environment, nitrate (NO3), is very soluble in water, the system is vulnerable to loss. If we liken the Iowa landscape to a glass filled to the brim with nitrogen, it’s easy to imagine disruptions causing the N to slosh out of the glass, escaping the farm into the stream network. Anyone involved with farming knows that N loss is closely linked to precipitation. And we all know that predicting weather is uncertain business. Even the pros are wrong much of the time. We try to manage inputs based on “average” weather. But average is not what we should expect in any given year; rather average is merely a mathematical construct that illustrates what the Iowa climate is like over time.
What we get from one year to the next may be far from average. About 2/3 of the N loss in Iowa occurs during only 1/3 of the year: April through July. We get about half of our annual precipitation during this period. If this four month period gets wetter relative to the rest of the year, all things being equal, N loss will increase. And as we have seen in previous segments of this series, the frequency of extreme precipitation events is increasing in Iowa.
To learn about N loss, we need to “follow the water.” Can we learn anything from evaluating how N loss has changed with extreme precipitation over time? It’s much harder to make conclusions about N than it is sediment and erosion. If a large rainfall follows a period of dry weather, there may be adequate water storage capacity in the soil and N may not move to the stream. On the other hand, even a moderate rain falling on wet soils can initiate or increase water flow through the soil profile and into tiles, causing large N delivery to streams.
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